It has been shown that tumors have developed numerous ways to escape tumor specific immune responses. These mechanisms will ultimately not only enhance tumor growth, but also impair the effect of immune based therapies in cancer. Myeloid derived suppressor cells represent a recently identified cell population, which has been shown to impair tumor specific immune responses both in mice and human. We have been able to identify a human counterpart of the initially only in mice described MDSC, which is characterized by the expression of CD14 and low/no levels of HLA-DR. We have tested different staining and isolation methods to determine relative and absolute MDSC frequencies in patients with cancer of the GI-tract. Our data shows that it is possible to use frozen PBMC isolated by Ficoll to determine MDSC frequencies in patients with cancer. This method will be used in future correlative studies.